A touch-sensitive screen enables the users to interact directly with what is displayed, and it becomes a popular user interface and is used widely in a variety of applications including automotive, aviation, marine, and consumer electronic applications, e.g. smartphone, tablet PC etc. There are a variety of touchscreen technologies that have different methods of sensing touch, including but not limited to, resistive type, surface capacitive type, projected capacitive type, infrared type, surface acoustic wave, and so on.
General speaking, mostly the methods of sensing touch are more or less susceptible to outside electromagnetic interference except the sensing touch methods of infrared and surface acoustic wave. While the present invention will be described in conjunction with a capacitive touch-sensitive apparatus, it will be understood that the descriptions are not intended to limit the present invention to this kind of touch sensing apparatus. On the contrary, a touch-sensitive apparatus using other types of touch sensing may apply the spirit of the present invention.
The sensing principle of projected capacitive touch apparatus are operated by measuring tiny changes in capacitance on an electrode due to a proximity or a touch (hereinafter called “proximity”) upon a touch apparatus caused by a human fingers or other objects. While electronic products are increasingly used in modern life, the effect of electromagnetic noise radiated by those electronic products is of growing concern. The interference voltages are coupled capacitively from electromagnetic noise sources that are both internal and external to the projective capacitive touch device. These interference voltages cause charge movement within the projected capacitive touch apparatus, which may be confused with the measured charge movement due to proximity upon the touch apparatus by a human fingers or other objects. The probability of identifying false touches by a touch control system of a projective capacitive touch screen will increase especially when the operating frequency of the projected capacitive touch apparatus is quite close to that of undesired electromagnetic noise.
FIG. 1 is a sinusoidal waveform diagram illustrating external electromagnetic waves in accordance with an embodiment of the present invention. As shown in FIG. 1, there are two example waveforms 110 and 102 which are sinusoidal noise with distinct frequency and amplitude. The interference voltages are coupled capacitively from electromagnetic noise sources 110 and/or 102 to a projective capacitive touch device during a proximity event caused by a user. For the projective capacitive touch device, the electromagnetic noise sources 110 and/or 102 are unknown. In other words, it is difficult to anticipate for the touch control system of the projective capacitive touch about the measured charge variation how much amount coupled from electromagnetic noise sources 110 and/or 102.
Accordingly, there is a need in the art about a proximity detection method, a touch-sensitive control device, and a touch sensitive apparatus against external electromagnetic interference, especially sinusoidal noise, to improve accuracy of proximity detection operation.